Among the many forms of cancer, cervical cancer is the second most common malignancy in women worldwide, exceeded only by breast cancer. In the United States, cervical cancer is the third most common neoplasm of the female genital tract. In 1994, 15,000 new cases of invasive cervical cancer and 55,000 cases of carcinoma in situ (CIS) were reported in the U.S. In the same year, an estimated 4,600 deaths occurred in the United States alone from cervical cancer. Recently, the incidence of pre-invasive squamous carcinoma of the cervix has risen dramatically, especially among young women. Women under the age of 35 years account for up to 24.5% of patients with invasive cervical cancer, and the incidence is continuing to increase for women in this age group. It has been estimated that the mortality of cervical cancer may rise by 20% in the next decade unless further improvements are made in detection techniques.
Early detection of cervical cancer, or of the pre-cancerous state called squamous intraepithelial lesion (SIL), can reduce the mortality associated with this disease. Currently, a Pap smear is used to screen for CIS and cervical cancer in the general female population. In a Pap smear, a large number of cells, obtained by scraping the cervical epithelium, are smeared onto a slide, which is then fixed and stained for cytologic examination. The Pap smear is unable to achieve a concurrently high sensitivity and high specificity due to both sampling and reading errors. For example, estimates of the sensitivity and specificity of Pap smears screening have ranged from 11-99% and 14-97%, respectively.
Furthermore, reading Pap smears is extremely labor intensive and requires highly trained professionals. A patient with an abnormal Pap smear indicating the presence of SIL is followed up by a diagnostic procedure called colposcopy, which involves colposcopic examination, biopsy and histologic confirmation of the clinical diagnosis. Colposcopy requires extensive training and its accuracy for diagnosis is variable and limited, even in expert hands. Moreover, diagnosis is not immediate. Thus, it would be desirable to provide a way to reduce cervical cancer rates by improving the methods for early detection. It also would be desirable to provide a diagnostic method that could improve the level of specificity and sensitivity, reduce the required skill level of the practitioner interpreting the results, and shorten the time that it takes to arrive at a diagnosis.
In vivo fluorescence spectroscopy is a technique which has the capability to quickly, non-invasively and quantitatively probe the biochemical and morphological changes that occur as tissue becomes neoplastic. The measured spectral information can be correlated to tissue histo-pathology to develop clinically effective screening and diagnostic techniques. By using automated data analysis techniques, there is the potential for an automated, fast, noninvasive and accurate pre-cancer screening and diagnosis system that can be used by non-experts.
Screening and diagnostic techniques for human tissue, and cervical pre-cancer tissue in particular, based on induced fluorescence spectroscopy have been developed relatively recently; see, for example, the following U.S. patents and patent applications, the teachings of which are hereby incorporated by reference: U.S. Pat. No. 5,699,795 to Richard-Kortums et al., Optical Probe for the Detection of Cervical Neoplasia Using Fluorescence Spectroscopy and Apparatus Incorporating Same; U.S. Pat. No. 5,697,373 to Richard-Kortums et al., Optical Method and Apparatus for the Diagnosis of Cervical Precancers Using Raman and Fluorescence Spectroscopies; U.S. Pat. No. 5,623,932 to Ramanujam et al., Diagnosis of Dysplasia Using Laser Induced Fluorescence; U.S. Pat. No. 5,612,540 to Richard-Kortums et al., Optical Method for the Detection of Cervical Neoplasias Using Fluorescence Spectroscopy; U.S. Pat. No. 5,562,100 to Kittrell et al., Method for Laser Induced Fluorescence of Tissue; U.S. Pat. No. 5,697,373 to Richard-Kortums et al., Optical Method and Apparatus for the Diagnosis of Cervical Precancers Using Raman and Fluorescence Spectroscopies; U.S. Pat. No. 5,612,540 to Richard-Kortums et al., Optical Method for the Detection of Cervical Neoplasias Using Fluorescence Spectroscopy; U.S. Pat. No. 5,421,339 to Richard-Kortums et al., Diagnosis of Dysplasia Using Laser Induced Fluorescence; U.S. Pat. No. 5,419,323 to Kittrell et al., Method for Laser Induced Fluorescence of Tissue; U.S. Pat. No. 5,345,941 to Rava et al., Contour Mapping of Spectral Diagnostics; U.S. Pat. No. 5,201,318 to Rava et al., Contour Mapping of Spectral Diagnostics; Ser. No. 467,993, Near-Infrared Raman Spectroscopy for In vitro and in vivo Detection of Cervical Precancers; Ser. No. 693,471, Method and Apparatus for the characterization of Tissue of Epithelial Lined Viscus; and Ser. No. 672,623, Spectroscopy Probe for In vivo Measurement of Raman Signals.
A problem with most optical and spectroscopic measurement systems and techniques is obtaining suitable signals indicative of the property to be measured. Contrast agents have been commonly applied to tissue in vitro and in vivo to enhance the optical return signal of illuminated tissue and thus aid in the extraction of diagnostically useful information from the sample. For example, techniques are commonly used to highlight cellular structures when using light microscopy to examine tissue samples. On a more gross level, sensitive differentiation between normal tissue and neoplasia in various tissue sites has been recently demonstrated through the use of 5-aminolevulinic acid induced protoporphyrin IX fluorescence.
Acetic acid is routinely used during colposcopy, a procedure involving examination of the cervix in situ with a low power microscope, to enhance differences between normal and diseased regions of the cervical epithelium. Areas which may develop into cervical cancer undergo a transient whitening (acetowhitening) visible to the naked eye. While the mechanism behind this phenomenon is not yet fully understood, it is commonly agreed that the higher nuclear density present in abnormal epithelium is a significant factor.
The inventors have determined that it would be desirable to provide a technique for the automatic spectroscopic detection of cervical pre-cancer that provides greater sensitivity and selectivity than prior techniques by enhancing the optical return signal in a manner that is indicative of abnormal tissue. The present invention provides such a technique.